The Future of Organic Photovoltaics by Katherine A. Mazzio and Christine K. Luscombe, reviews current scientific progress made in the field of organic photovoltaics (OPV), relating directly to:
- Common types of OPV device structures;
- Popular electron donor and acceptor materials;
- Common active layer morphology optimisation techniques.
Here I will present a summary of particular strengths and limitations displayed in the review by Mazzio et al. (2009) .
Can a single layered OPV device demonstrate reasonable efficiency?
Mazzio et al. (2015) presents an informative, chronological summary of device structural characteristics, limitations and strengths . However, a slight discontinuity was reported in the authors’ description of single layered devices, when compared to current research. Mazzio, et al. (2015) suggests that PCEs in the order of only ca. 0.1% have been achieved, to date, for single layered devices, owing to, limitations in device arrangement . The device arrangement for a single layered OPV is displayed in Figure 1 below . Zhang, Q. et al. (2015) recently reported a power conversion efficiency, PCE of ca. 8% for polymer fullerene solar cells . Greater power conversion efficiencies were also reported by Liao, S, H. et al. (2014); a polymer fullerene single cell containing a cathode doped with ZnO, achieved a PCE of ca.10.31% . From the cited material it can be presumed that single layered devices can produce power conversion efficiencies of greater magnitude than those cited by Mazzio. et al. (2015). Recent developments in single junction OPVs clearly discredit the inaccurate information presented by Mazzio. et al. (2015) in this review.
Limitations of the Bulk Heterojunction
Careful selection of device architecture could improve OPV performance [1, 5]. Mazzio. et al. (2015) reported limitations in the stability of Bulk Heterojunction (BHJ) OPVs. The authors notably suggested an alternative device architecture that may overcome these limitations; an Inverted Bulk Heterojunction, as illustrated in Figure 2 [1, 5, 6]. Mazzio. et al. (2015) reported that the acidic nature of PEDOT: PSS could contribute to BHJ device degradation . Mazzio. et al. (2015) suggested that inverted architectures typically replaced PEDOT: PSS with a low work function metal such as zinc oxide, ZnO . However, Hau, S, K. et al. (2008) reported an excellent air-stable inverted BHJ device containing PEDOT:PSS alongside a silver, Ag electrode . Hau, S, K. et al. (2008) suggested that the superb stability displayed in tests is owed to, PEDOT: PSS and Ag limiting device oxidation . In spite of, the suggestion made by Mazzio, et al. (2015); PEDOT: PSS must be removed in order to improve stability, Hau, S, K. et al. demonstrates that the PEDOT: PSS layer can improve device stabilities. This improvement in stability in inverted BHJ OPVs make this device an attractive alternative when compared to regular BHJ device architectures. Mazzio. et al. (2015) highlighted the importance of careful selection of device architectures, in order to minimise limitations.
Are small molecules contenders to polymers?
Limitations of polymer active layers were expressed by Mazzio. et al. (2015) concerning: variations in batch to batch properties, end group variations and dispersity . Small molecules (displayed in Figure 3) are presented as potential future alternatives and contenders to polymers, not only overcoming the mentioned limitations but also exhibiting greater hole and electron mobilities [1, 7].
Small molecules have come a long way, since, Mazzio. et al. (2015) reported their highest PCE’s as ca. 6% with Zhang, Q. et al. (2015) reporting a remarkable efficiency of ca. 9% for a single layered OPVs  and tandem devices recording efficiencies of >12 . Comparatively, polymer based tandem OPVs demonstrate efficiencies of ca. 10% also, PCEs of ca. 9% for a single layered device . From these studies, one can note the improvement in the PCE generated from tandem device architectures, for both small molecule and polymer active layers [3, 9] . From the perspective of power conversion efficiencies alone Mazzio. et al. (2015) is correct to describe small molecules as competitors to polymers, as they have achieved PCEs in a similar range. The reader that is interested in tandem device architectures should read the article by You, J. et al.(2012), where PCES of >10% are recorded for OPVs or Sista, S. et al. (2011) .
Morphology Optimisation Procedures
In order to optimise small molecule and polymer active layers, Mazzio. et al. (2015) provided an extensive description of optimisation procedures that alter active layer morphologies and improve device efficiencies . The authors explore the effect of; solvent annealing, thermal annealing and the addition of solvent additives on the active layer morphology of materials, using peer-reviewed sources . The authors discuss, thermal annealing; the process by which a film can be heated above its glass transition temperature, allowing the material to re-orientate itself . They go on to discuss the effect of different types of annealing; post-annealing and pre-annealing . In post-annealing, the sample is thermally annealed after cathode deposition onto the device, whereas, pre-annealing describes thermal annealing prior to cathode deposition . Mazzio. et al. (2015) suggests that these techniques can be used to improve device efficiencies . This is supported by a study carried out by Yi, Z. (2014) on the pre-annealing of small molecule, that increased the dark current, reduced the open circuit voltage, hence, improved the PCE . An experimental study conducted by Yang, X. et al. (2012) supported this further; post annealing was found to enhance light absorption (as shown in Figure 4) and increase hole mobilities, furthermore, improving PCEs . In addition to demonstrating that device architectures can influence power conversion efficiencies the writers also portray that annealing a sample can also optimise OPV properties.
Importantly, the authors provide an alternative to thermal annealing, for materials such as PCPDTBT that does not respond well to thermal annealing; the addition of alkane dithiols . Mazzio. et al. (2015) report an improvement in the nanomorphology of PCPDTBT/PCBM on the addition of 1, 8-di-iodooctane; through increased PCBM domain development . Albrecht, S et al. (2012) also found that the addition of solvent additives drove phase segregation, in active layers containing PCPDTBT, as displayed in the energy filtered transmission electron microscopy, EFTEM surface images; Figure 5 below .
However, the authors do not report the enhanced device properties, that result from improved nano-morphologies of PCPDTBT; improved mobility , reduced field dependence  improved short circuit current , enhanced optical absorption [13, 14]. Both thermal and solvent additive morphological optimisation techniques were found to drive phase segregation and improve various properties of OPVs [1, 8, 12, 13]. Yet, the discussion of morphology optimisation procedures by Mazzio. et al. (2015) will provide valuable information to new researchers, to the popular field of OPVs, on how power conversion efficiencies can be enhanced.
Conclusively, from the discussion above it is apparent that the review by Mazzio, et al. (2015) demonstrates an informative insight into potential routes of optimisation of OPV efficiencies. The review allows the reader to recognise the influence that components in an OPV can have on the device efficiency. Overall, this review article was mostly consistent but in certain areas the information presented was inaccurate, perhaps, owing to negligence.
Predominantly, Mazzio, et al. (2015) presented a well-structured review suitable for those new to the field. Moreover, for future work I would suggest that the writers present a literature review on the current status and future applications of small molecules suitable to those new to the field, since, these are limited.
By Naeema Ebrahim
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